1. Field of the Invention
The present invention relates generally to DC-DC converters and, more particularly, is directed to a DC-DC converter for use as a dc power source for an electronic flash of a camera or the like.
2. Description of the Prior Art
As a dc power source for an electronic flash of a camera, there is employed a DC-DC converter shown in FIG. 1 in the prior art. Referring to FIG. 1, there is provided an oscillator 1 which is formed of an astable multivibrator. This oscillator 1 oscillates at a frequency of, for example, 22 kHz. A battery 2 of, for example, 24 V is used as a dc power source for the oscillator 1. The oscillation output signal from the oscillator 1 is supplied to a primary winding 3a of a transformer 3. One and the other ends of a secondary winding 3b of the transformer 3 are respectively connected to one and the other input ends of a full-wave rectifying circuit 4 which comprises four diodes 4A, 4B, 4C and 4D connected with the polarities as shown in FIG. 1. One output terminal 4a of the full-wave rectifying circuit 4 is connected through a fly-wheel diode 5 to the other output terminal 4b of the full-wave rectifying circuit 4. The output terminal 4a of the full-wave rectifying circuit 4 is connected through a series circuit formed of a parallel circuit of a capacitor 6 and a resistor 7 and a reverse-blocking diode 8 forming a spike voltage removing circuit to one output terminal 9a. The output terminal 4a of the full-wave rectifying circuit 4 is further connected to the output terminal 9a through a choke coil 10. The other output terminal 9b is led out from the other output terminal 4b of the full-wave rectifying circuit 4, and a smoothing capacitor 11 is connected between both output terminals 9a and 9b.
In the thus arranged conventional DC-DC converter, the oscillator 1 generates an oscillation signal of which the peak value is 24 V. This oscillation signal of 24 V is increased to, for example, 500 V by the transformer 3. Then, the oscillation signal having the peak value of 500 V is rectified by the full-wave rectifying circuit 4. The rectified signal from the full-wave rectifying circuit 4 is smoothed by the smoothing capacitor 11 to thereby produce a dc voltage of, for example, 460 V.
The known DC-DC converter shown in FIG. 1, however, cannot remove the following defects. That is, the full-wave rectifying circuit 4 produces across its output terminals 4a and 4b comparatively large spike voltages at the leading edge and the trailing edge of the square wave shown in FIG. 2A. Since in the example of the conventional DC-DC converter shown in FIG. 1 the negative spike voltage is removed by the capacitor 6, the resistor 7 and the diode 8 and only the positive voltage in FIG. 2A is supplied to and smoothed by the smoothing capacitor 11, the negative spike voltage cannot be utilized effectively. This negative spike voltage causes the capacitor 6, the resistor 7, the diode 8 or the like to generate heat and, also loses the power.